1. Field of the Invention
The present invention relates to an electret condenser microphone having excellent heat resistance and also relates to a method of producing the electret condenser microphone.
2. Description of the Related Art
A conventional electret condenser microphone (hereinafter abbreviated as “ECM”) has a diaphragm as a vibrating plate and an electret layer (electrically charged resin layer) that are opposed to each other to constitute a capacitor, which is used as a vibration detecting device.
A method of producing such a conventional ECM is disclosed, for example, in Japanese Patent Application Publication No. 2002-345087. According to this method, an electret layer, which is an electrically charged resin material, is prepared by forming a resin layer on a metallic backplate substrate serving as a detecting electrode, and implanting an electric charge into the resin layer (see FIG. 3 in the above-mentioned publication). Alternatively, a backplate is film-formed on a backplate substrate of a resin or ceramic material, and the above-described electret layer is formed on the backplate (see FIG. 1 in the above-mentioned publication).
When used in household devices and the like, the ECM having the electret layer is, generally, soldered to a motherboard having other electric elements mounted thereon. In order to reduce ECM mounting costs, solder mounting using a reflow oven is demanded. In the solder mounting using a reflow oven, the ECM is subjected to preheating at 160° C. to 180° C. for about 100 seconds, followed by heating at 250° C. for about 10 seconds. Under the high-temperature conditions, the electric charge implanted in the electret layer decreases, with the result that the ECM may become unable to perform its function as a microphone. In other words, the ECM has the problem that the electret layer is inferior in heat resistance.
Some propositions have heretofore been made to solve the above-described problem. For example, Published Japanese Translation of PCT International Publication for Patent Application No. 2001-518246 discloses an ECM that uses an inorganic silicone resin as an electret material in place of an organic resin material which is inferior in heat resistance. Silicone resins, however, are higher in cost than organic resin materials.
Japanese Patent Application Publication No. 2000-32596 discloses an ECM that enables solder mounting using a reflow oven by improving a conventional organic electrically charged resin layer (electret layer). In this ECM, an organic resin material for constituting an electret layer is fusion-bonded to a metal plate constituting a backplate substrate. Then, the metal plate is subjected to high-temperature annealing at about 200° C. for about 1 to 6 hours, followed by electric charge implantation, thereby forming a high heat-resistant electrically charged resin layer.
Japanese Patent Application Publication No. 2005-191467 discloses a method of forming a resin layer for electric charge implantation which is made of a heat-resistant resin material, i.e., polytetrafluoroethylene (hereinafter abbreviated as “PTFE”) and comprises two or more layers, by successively fusion-bonding PTFE films onto a metal plate. In this method, in order to improve the inferior adhesion between the metal plate and the PTFE film, the first PTFE film is fusion-bonded to the substrate at a high temperature of 370 to 390° C., and the second PTFE film is fusion-bonded onto the first layer of PTFE at a temperature lower than the fusion-bonding temperature for the first film, i.e. at 330 to 350° C.
In addition, the above-mentioned Patent Application Publication No. 2005-191467 discloses methods of improving the adhesion between the PTFE film and the metal plate. It is stated as one of the methods that an adhesive layer of a thermoplastic resin is provided between the metal plate and the PTFE film. Paragraph [0009] in this publication, however, states that “the provision of the adhesive layer makes it impossible to obtain the desired characteristics of the fixed electrode and degrades the electric charging characteristics”. The reason for this may be as follows. If the metal plate is die-cut into a desired electrode shape after a PTFE film has been bonded thereto through a thermoplastic resin adhesive layer, the adhesive layer is strained by the impact force of die-cutting, which exerts an adverse effect on the characteristics of the PTFE film as an electret layer.
In recent years, it has been noted that the above-described PTFE film and other fluorine-containing resin materials are excellent in heat resistance and moisture resistance, and there has been an increasing demand for fluorine-containing resin materials as moisture-proof sealants. Under these circumstances, there is a commercially available adhesive-backed fluorine-containing resin film formed by stacking an adhesive on a fluorine-containing resin material having a surface activated by surface treatment with an alkaline metal amide in liquid ammonia. This adhesive-backed fluorine-containing resin film is used as a tape material under adverse environmental conditions such as high-temperature and high-humidity conditions.
As has been stated above, one technique disclosed in the foregoing related arts features that a sheet of electret material is fixedly secured by fusion bonding to the surface of a metal plate, and the metal plate having the electret material sheet is die-cut by press working to form a backplate in the shape of an electrode. This technique suffers from the following disadvantages.
Because a sheet of electret material is fusion-bonded directly to a metal plate to be formed into an electrode, if there is a large change in temperature after the electric charge implantation process, thermal expansion of the metal plate influences the sheet of electret material so that molecular motion occurs in the electret material, resulting in the implanted electric charge to disappear from the electret material sheet.
In addition, metal cutting process such as press working performed to shape the metal plate causes internal strain in the electret material sheet fixedly secured to the metal plate. This causes the electrically charged condition to become unstable, resulting in a decrease in the electric charge implanted in the electret material sheet.
Japanese Patent Application Publication No. 2002-345087 discloses in FIG. 1 thereof an arrangement in which a backplate is film-formed on a backplate substrate of a resin or ceramic material, and an electret layer is formed on the backplate. There is, however, no specific explanation of a method of film-forming the electret layer.
Regarding the method of minimizing the decrease of the implanted electric charge by increasing the heat resistance of the electret material, the technique disclosed in Japanese Patent Application Publication No. 2000-32596 is basically an annealing technique applied to the resin material. Therefore, the material needs to be allowed to stand for a long period of time under high-temperature conditions. This involves the disadvantage of an increase in the production time and also a problem in terms of the stability of the product due to variations in the time control and temperature control.
The technique disclosed in Japanese Patent Application Publication No. 2005-191467 needs to fusion-bond at least two sheets of electret material onto the upper side of a metal plate at different temperatures. The process of performing a plurality of fusion-bonding steps at different temperatures is inferior in productivity. This technique finally requires machining to cut the metal plate.
Under these circumstances, the present inventor studied to find a method of producing an ECM capable of reflow mounting and excellent in heat resistance without degrading productivity and, as a result, took notice of the above-described commercially available adhesive-backed fluorine-containing resin film surface-treated with an alkaline metal amide in liquid ammonia. That is, the present inventor has found an ECM production method using the adhesive-backed fluorine-containing resin film, which makes the best use of the excellent electric charging characteristics thereof.